This invention relates to detector arrays usable for optical imaging and surveillance purposes. It constitutes a "breakthrough" in this art because it succeeds in combining the benefits of a staring sensor array with the benefits of a scanning sensor array. And, additionally, it provides benefits which neither staring nor scanning arrays have accomplished. A staring array is usually a two-dimensional focal plane mosaic of photodetectors each of which continually "sees" a given pixel, i.e., an area of observation representing one unit of resolution. A scanning array is usually a line array of photodetectors, which are moved back and forth across a row, or column, of pixels.
In comparing scanner and starer capabilities, those possessed by one, but not by the other, may be treated as benefits of one or as deficiencies of the other. The following is a list of the primary scanner "deficiencies" in this context:
(1) Clutter limited performance;
(2) Mechanical complexity;
(3) Large transient pulses (when compared to true signals) from ionizing radiation;
(4) Difficult detector requirements;
(5) Large optics;
(6) Complex signal processor.
The following is a list of the primary starer "deficiencies" in this context:
(1) High detector count and operability requirements;
(2) Difficulty of multi-color operation;
(3) 1/f noise;
(4) Line of sight jitter and drift;
(5) Radiometric accuracy.
Three major additional deficiencies which are dealt with by the present invention, but not by either scanner or starer systems, are: (1) susceptibility to an overriding signal; (2) detector and electronics outage; and (3) discrimination between target and background regardless of their relative motion or intensity.
In a broad sense, the present invention deals with the problem of distinguishing a specific element, or "target", in the viewed scene, from its background. One type of difficulty exists when the background is essentially stationary, but the target is moving. The stationary background might, for example, be a cloud. Another, very different type of difficulty exists when the "target" appears to be essentially stationary, but the background appears to be moving. This situation may occur where the detector system is onboard a moving vehicle, and the target is coming directly toward the vehicle. Or, a further possibility exists where neither the detector nor the target appears to be moving, but the distance between them is changing.
The assignee of the present application has worked in both the starer and scanner fields, but primarily in the former. A plurality of prior applications, also assigned to the assignee of this application, relate to this general field of development.
Some of those applications deal with detector-supporting structures, and others deal with detector-connected circuitry. For example, U.S. Ser. No. 572,802, filed Jan. 23, 1984 as a continuation of U.S. Ser. No. 187,787, relates to a "focal plane" structure having a multiplicity of stacked semiconductor chips, which extend in planes perpendicular to the focal plane, and which carry circuits providing detector-contacting electrical leads at the focal plane. Each detector may (as shown in that application, and also in U.S. Pat. No. 4,490,626, issued Dec. 25, 1984, and in U.S. Ser. No. 558,099filed Dec. 5, 1983) send its individual signal through a separate circuit comprising a pre-amplifier and a bandpass filter; and a multiplexer may combine the pre-processed signals from a number of such detector circuits. All of this circuitry for a multiplicity of detectors may be located on a single chip; and the stacked chips may provide circuitry for a dense array of detectors.
The present invention might be used with the stacked chip technology; or it might be used with other types of focal plane staring detector arrays.
It is well known that staring two-dimensional detector arrays have a major sensitivity advantage over scanning linear arrays. Another advantage of staring arrays is their ability to ignore non-moving, or slow-moving, objects, such as clouds. Because of the staring, the detector is "aware" that no significant change has occurred, i.e., background objects do not have the potentiality of "confusing" the array. On the other hand, since a scanning array creates its own motion, a non-moving object is "perceived" by a scanner as a signal change. The electronics must process this information, in order to determine that the object is not moving.
But any sensing system having an extremely large number of staring detectors is susceptible to certain problems. One significant problem is the extreme difficulty of obtaining adequate performance from every detector. Another significant problem, in space applications, is the potential effect of gamma rays in causing over-stimulation of any detectors they affect. A third problem is the difficulty of reducing pixel size, because of the mechanical limitations on the closeness of detectors to one another, a problem which is more difficult to solve in a two dimensional array than in a line array. And finally, a staring array provides no solution to the difficulty of separating a target which appears to be stationary from a background which appears to be moving.